In recent years, water-absorbent resin, which is a type of hydrophilic resin, has been widely used as, for example, a constituent material of sanitary materials for disposable diapers, sanitary napkins, pads for incontinence, etc., in order to prevent contamination of clothing, etc., by bodily fluids such as urine and blood, by absorbing and retaining these bodily fluids.
Further, recently, in order to make such sanitary materials thinner and improve their performance, the trend is to decrease the amount of fiber base materials (such as pulp) and increase the amount of water-absorbent resin used therein. Accordingly, further improvement of the water absorbing performance of such water-absorbent resin is needed.
Such water-absorbent resins can be obtained by polymerizing monomers such as acrylic acid, sodium acrylate, etc. in an aqueous solution in the presence of a crosslinking agent, yielding lumps of a hydrogel polymer, which are pulverized to a suitable particle size, dried, and then further pulverized as necessary.
One conventional method of pulverizing the foregoing hydrogel polymer is pulverizing by feeding between a two-shaft roller-type cutter.
However, when pulverizing the hydrogel polymer using the foregoing conventional method, even if the foregoing hydrogel polymer is fed into the roller-type cutter, it is not easy to bite into the hydrogel polymer, because it is a semi-solid gel having elasticity, and the hydrogel polymer may be kneaded during pulverizing. For this reason, if the foregoing conventional method is used, the hydrogel polymer is subject to great mechanical external force, and there are cases in which, for example, crosslinked polymer chains are broken, thus increasing the content of water-soluble components therein.
Further, in order to improve the water absorbing performance (especially absorbing capacity and absorption rate) of the water-absorbent resin, the foregoing hydrogel polymer is often made to contain bubbles by polymerizing the constituent monomers in an aqueous solution in the presence of a crosslinking agent. In this case, with the foregoing conventional pulverization method, the bubbles contained in the hydrogel polymer are squashed and reduced in number. Accordingly, in a water-absorbent resin obtained in this way, since the reduced number of bubbles leads to decreased surface area, it may not be possible to ensure sufficient liquid infiltration spaces necessary for movement of the aqueous liquid, thus impairing permeability to and dispersion of the aqueous liquid. For this reason, a problem with water-absorbent resins obtained by the conventional methods is that, during the production process, not only is the content of water-soluble components increased, but performance, such as absorbing capacity, is also impaired. These problems are not limited to water-absorbing resins, but are common to all hydrophilic resins.